Method of motion compensation with synthetic rope

ABSTRACT

A method of providing motion compensation of a subsea package with a synthetic rope comprising attaching the synthetic rope to the subsea package, supporting a first gripper with a wire rope from a winch capable of motion compensation control characteristics and gripping the synthetic rope with the first gripper, supporting a second gripper with a second wire rope, and repeating the following sequence: lowering the first gripper, the synthetic rope, and the subsea package a first distance, gripping the synthetic rope with the second gripper, releasing the first gripper from the synthetic rope, raising the first gripper the first distance, gripping the synthetic rope with the first gripper, releasing the second gripper from the synthetic rope, such that when the subsea package is lowered proximate the subsea landing location the winch capable of operating with motion compensation characteristics can operate to compensate for the vessel motion and smoothly lower the subsea package to the subsea landing location.

TECHNICAL FIELD

This invention relates to the method of providing motion compensationoperations for synthetic rope especially as relates to deep waterlifting and lowering operations.

BACKGROUND OF THE INVENTION

Offshore equipment packages are often placed on the ocean floor bylowering on a cable to the desired location. Especially when one packageis to be landed on the top of another package, the surge of the vesselon the surface can cause damage to the subsea equipment being landed andthe equipment being landed on as they impact one another. Often timesthe packages must be landed securely before hydraulic connectors canjoin them together.

Motion compensation is used to detect the motion of the vessel and paythe supporting line in and pull it back by operating the winch as thevessel heaves so that the package near the seafloor can remainvertically stationary and then slowly be lowered to engagement. Thedefault material for the lifting line is a high strength steel cable.

Running heavy subsea packages to depths such as 10,000 feet have theproblem that when that much steel cable is deployed, the majority of thecable's strength is consumed in simply holding its own weight as steelcable is relatively heavy.

If a near neutrally buoyant synthetic rope is utilized, most of thecable's strength is retained to lower or lift the subsea load. Syntheticrope is basically a rope made of a high strength form of plastic, so itis relatively light per unit volume.

Wire cable or synthetic rope can be handled on single drum winches orwinches which are comprised of a multiplicity of powered sheaves orpulleys. In either case the wire rope or synthetic rope is flexed oncein lowering, but is repeatedly flexed during motion compensation times.This is of little consequence in wire cable, but the sliding friction(internal and on the sheaves) of the synthetic rope will generate highheat, potentially to the point of destroying the synthetic rope due tomelting.

The capability to lower subsea packages to the seafloor with a syntheticrope under motion compensation control would allow substantiallyexpanded deep sea lifting and lowering capabilities as contemporarywinching methods for synthetic rope has the potential for damaging thecritical synthetic rope during motion compensation operations.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide a method of providing motioncompensation for a synthetic rope without sliding friction heatgeneration.

A second object of this of this invention is to provide method of motioncompensation for a synthetic rope without internal heat generation.

A third objective invention is to provide a method of deep sea liftingcapacity without the load capacity being reduced as a function of thedepth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a motion compensation system of thisinvention.

FIG. 2 is a perspective view of a conventional winch for synthetic rope.

FIG. 3 is a half section illustrating how conventional slips with sharpteeth work.

FIG. 4 is a half section of a conventional slip assembly to show whatwould happen if the sharp teeth were changed to a smooth surface.

FIG. 5 is a perspective view of a slip assembly illustrating doublewedging for failsafe support.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a pictorial view of a motion compensationsystem 10 is shown being landed on a boat 12 having a storage area 14for storing a length of synthetic rope 16. The synthetic rope 16 canalso be stored on a reel if desired. The synthetic rope 16 is showncoming out of the storage bin at 18, going over a sheave 20 which issupported in a mast 22, continues downwardly at 24 and 26, and connectsto subsea package 28 at connection 30.

An upper winch 40 is provided with a wire rope 42 going over sheave 44,with the wire rope continuing downwardly at 46 and connects to gripper48 at connection 50.

A lower winch 60 is provided with a wire rope 62 going over sheave 64,with the wire rope continuing downwardly at 66 and connects to gripper68 at connection 70.

As is illustrated, the synthetic rope 16 passes over the sheave 20 withno significant loading, but rather the load of the synthetic rope 16 andthe subsea package 28 is carried by either gripper 48 or 68. Gripper 48and 68 have steel cables which will carry the load over sheaves 44 and64 respectively. As will be discussed, steel cables can be utilized topass repeatedly over the sheave with minimal heat build-up and can bewrapped onto the winches tightly to support the loadings. Synthetic ropewill generate both substantial and damaging heat if run over a syntheticrope winch and cannot be wrapped tightly on a conventional winch as theouter layers will “knife” in between the inner layers.

Winch 40 lowers its gripper 48 a distance and stops, presume for examplefifty feet. The winch 60 will have raised its gripper 68 fifty feet. Atthat time gripper 68 is engaged with the synthetic rope 16 and thengripper 48 is released. Gripper 68 is now lowered fifty feet and gripper48 is raised fifty feet. The “hand over hand” process is continued withthe subsea package 28 being lowered one hundred feet each cycle. Thismeans that the subsea package 28 can be lowered ten thousand feet to theseafloor with two steel cable winches which have a working length of notmuch more than fifty feet.

Referring now to FIG. 2, a conventional synthetic rope winch 78 isillustrated with synthetic rope 16 wrapped around two sheave groupings80 and 82 which are mounted on support stands 84 and 86 and are poweredby motors 88 and 90. The Sheaves of sheave groupings 80 and 82 are on acommon axle (now seen) and therefore all turn the same speed. Each ofthe sheave groupings illustrated have ten sheaves.

The tension on synthetic rope 16 is indicated as a small load at 92 andas a high load at 94 after passing over the twenty sheaves. This meansthat the motors 88 and 90 have powered the sheave groupings 80 and 82respectively and the friction between the synthetic rope 16 and thesheaves has caused a tension to be pulled. If we presume the tension at92 is effectively zero and the tension at 90 is one hundred thousandpounds, it means on average each sheave has increased the tension of thesynthetic rope by five thousand pounds. As synthetic rope is relativelyelastic (has a relatively lower spring coefficient), each time fivethousand pounds is added to it, the synthetic rope stretches a littlemore. There are twenty sections of rope 96 between the sheave groupings80 and 82, each of which is under a different tension. This means thatas the synthetic rope begins contact with a sheave such as at 98 untilit loses contact with the sheave such as at 99, the tension changes. Asthe tension changes in the elastic synthetic rope, the length changes.If at any point the synthetic rope is travelling at exactly the samespeed as the contact surface on the sheave, it will be travelling at adifferent speed at all other locations. This means that sliding frictionis generated around most of the contact surfaces. If you can imaginethat at only one point on one sheave the tension is correct for thesynthetic rope to be travelling at the same speed as the surface of thesheaves, then at all other points on all the other nineteen sheaves thespeed will not be matched. At all other contact points sliding will becontinuously occurring, with the resulting friction heat generation.

You can imagine that with all this sliding going on with high loads,i.e. one million pounds, a lot of heat generation is occurring. When youare doing motion compensation, which means you are going back and forthover the same section of synthetic rope, this generated heat has thepotential of building up to the point of damaging the synthetic rope.

Referring now to FIG. 3, the difficulty of supporting the synthetic ropewith a gripper such as indicated at 48 or 68. The synthetic rope isrelatively slippery and is difficult to grip. If it is simply gripped,there is a chance that the grip will slip sooner or later and the ropeand subsea package will be lost. It is prudent that a “failsafe” gripwill be provided. Failsafe generally means that the higher the load, thehigher the gripping force. Conventional failsafe slips for pipe 120 areshown with a steel pipe section 122 about centerline 124, slip segments126 with sharp teeth 128, and bowl 130. The coefficient of friction 132between the bowl 130 and the slip segments 126 is in the range of 0.10.The sharp teeth 128 literally bite into the pipe so exact coefficient offriction 134 is questionable, however, it is conventional to use 0.5 asthe coefficient of friction to calculate with. This means that when aload 136 is imposed on the pipe and is resisted by a foundation support138, the slip segments 126 will slip at 132 rather than 134, meaning itwedges more tightly when the load increases—it has “failsafe” support.

Referring now to FIG. 4, it can be seen what happens if the sharp teethare removed from the slip segments so that they do not cut and damagethe synthetic rope. Slip segments 140 have a smooth surface 142 whichwould have a comparable coefficient of friction 144 of 0.10, or evenless as the synthetic rope is slicker than the steel pipe 140 shown. Atthis point there would be at least a fifty percent chance that the pipewill slip rather than the slip segment sliding down the bowl taper to atighter grip. The eight degree surface 146 between the slip segments 126and the bowl 130 add a vertical component resisting slip segmentmovement, insuring that the pipe will slide in the slips rather than theslip segments sliding down with a tighter grip.

Referring now to FIG. 5, a gripper method 150 is shown and as describedin U.S. Pat. No. 6,820,705. Smooth surfaces 152 to engage the syntheticrope are provided on four slip segments 154. Two eight degree surfaces156 in bowl 158 take the place of the eight degree conical bowl in theconventional slips. When the two eight degree tapered surfaces 160engage the surfaces 156, the primary force to grip the synthetic rope isprovided. In this case the slip carriers 162 have four wedging surfaces164 which engage mating surfaces 164 on slip segments 154. This meansthat the force from the eight degree tapered surfaces 156 is wedged oramplified by the surfaces 162 such that the amplified friction forceagainst the synthetic rope will exceed the resisting force from comingup from surface 156. This means that the slip carriers 162 and the slipsegments 152 will slip down into increased grip as the load increasesmaking them “failsafe”.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

That which is claimed is:
 1. A method of providing motion compensationof a subsea package with a synthetic rope comprising attaching saidsynthetic rope to said subsea package, supporting a first gripper with awire rope from a winch capable of motion compensation controlcharacteristics and gripping said synthetic rope with said firstgripper, supporting a second gripper with a second wire rope, andrepeating the following sequence: lowering said first gripper, saidsynthetic rope, and said subsea package a first distance, gripping saidsynthetic rope with said second gripper, releasing said first gripperfrom said synthetic rope, raising said first gripper said firstdistance, gripping said synthetic rope with said first gripper,releasing said second gripper from said synthetic rope, such that whensaid subsea package is lowered proximate the subsea landing locationsaid winch capable of operating with motion compensation characteristicscan operate to compensate for the vessel motion and smoothly lower saidsubsea package to said subsea landing location.
 2. The method of claim 1further comprising said first slips and said second slips are failsafe.3. The method of claim 1 further comprising said first slips and saidsecond slips do not have a friction enhancing tooth profile.
 4. Themethod of claim 1 further comprising said synthetic rope is at least tentimes as long as said first wire rope.
 5. The method of claim 1 furthercomprising said first gripper is engaged and released by remote control.6. The method of claim 1 further comprising said second gripper isengaged and released by remote control.
 7. A method of providing motioncompensation of a subsea package with a synthetic rope comprisingattaching said synthetic rope to said subsea package, supporting a firstgripper with a wire rope from a winch capable of motion compensationcontrol characteristics and gripping said synthetic rope with said firstgripper, supporting a second gripper with a second wire rope from awinch capable of motion compensation control characteristics, andrepeating the following sequence: lowering said first gripper, saidsynthetic rope, and said subsea package a first distance, gripping saidsynthetic rope with said second gripper, releasing said first gripperfrom said synthetic rope, raising said first gripper said firstdistance, lowering said synthetic rope with second gripper a seconddistance, gripping said synthetic rope with said first gripper,releasing said second gripper from said synthetic rope, raising saidsecond gripper said second distance, such that when said subsea packageis lowered proximate the subsea landing location either of said winchescapable of operating with motion compensation characteristics canoperate to compensate for the vessel motion and smoothly lower saidsubsea package to said subsea landing location.
 8. The method of claim 7further comprising said first slips and said second slips are failsafe.9. The method of claim 7 further comprising said first slips and saidsecond slips do not have a friction enhancing tooth profile.
 10. Themethod of claim 7 further comprising said synthetic rope is at least tentimes as long as said first wire rope.
 11. The method of claim 7 furthercomprising said first gripper is engaged and released by remote control.12. The method of claim 7 further comprising said second gripper isengaged and released by remote control.